Love‐ and Rayleigh‐wave microseisms with a dominant period of about 20 s have been clearly detected in Japan. Because their arrival direction was almost the same in any part of the Japanese islands, they are expected to have propagated as plane waves from a very distant source. We estimated the source area of the microseisms by multiple array analysis of data from the Hi‐net, high‐sensitivity accelerometers in Japan and the broadband seismic array at Gräfenberg, Germany. The source was distributed to be in the North Atlantic and migrated from north of the British Isles to the Norwegian Sea in a period of one day. This migration coincided with the propagation of ocean swells. Our results indicate that ocean swells in the North Atlantic can be a major source of seismic noise with a period of 20 s in Japan. Our analysis also suggests that the source areas of the Love and Rayleigh waves were different. It is hoped that such observations will be helpful in understanding the excitation mechanisms of microseisms.